A stream of high viscosity material can be discharged uniformly and in a shaped manner using screw machines. Plastic and viscoelastic compositions can be contaminated by foreign bodies such as, for example, particles of dirt or rust. Undesirable solids are filtered out in a filtering process with the aid of fine sieves which allow the plastic or viscoelastic composition to flow through but which retain the particles of solids. Sieve changing devices with round sieve plates are known. They are mounted on the extruder flange with suitable adaptors and inserts. The main body is provided with a hydraulically activated sliding plate and a sealing system. In order to change the sieves, a cassette with two sieves arranged in it is pushed by means of a cassette guide under a high pressure load by a hydraulic device transversely through the stream of melt, the first, already soiled sieve coated with particles of foreign matter being removed from the stream of melt and the second new sieve being placed in the stream of melt. The sieve changing process takes place under the high static axial pressure of the sliding faces so as to prevent melt from issuing laterally, as much as possible. A sieve changing device in a two-shaft kneading disc screw press is also known. However, the cross-section of the sieve is also round in this case. A "figure of eight-to-round member" which is fitted between the tips of the screws and the perforated plate has been developed for it. The cross-section of flow must be altered to the cross-section of the respective die shape, for example a perforated ridge, after the round plane of the sieve. It has been found that, in screw processes, thermally sensitive polymers are so highly stressed at the end of a screw process that the volume of the "figure-of-eight member" of the sieve devices and of the die must be considered as a dead space. Although continuous, kinematically compelling self-cleaning and forced feed prevails in the region of the screw shafts, in particular in the case of closely intermeshing multiple shaft screws which rotate in the same direction (see, for example, H. Herrmann, Schneckenmaschinen in der Verfahrenstechnik, Springer, Berlin, Heidelberg, New York, 1972, page 120 et seq), the material is conveyed in a laminar flow but without mechanical force and self-cleaning in the space between the end of the screw shaft and the die openings, merely by means of the pressure produced by the screw. This transitional space from the screw contour to the die openings is unavoidable. It results in a broader residence time spectrum and gives rise to the risk of damage to the material.